As an EU citizen you will need to obtain a Foreigner’s Identification Number (NIE) before you start any job in Spain, no employer can (legally) employ you without one.
Put in to Google ‘NIE form EX14’ and you will see .pdf links on the search results (should be 2nd one down) to the form you need to print off, complete, then take to your nearest National police station with a foreigners’ department (Oficina de Extranjeros), when your in Spain, along with your passport and several passport photos.
Usually once you arrive at the police station you can expect at least several hours of waiting in line, and they close at 2pm so it helps to get there as early in the morning as possible. Once they have spoken to you they will give you a payment form which you need to take to the nearest bank to pay the fee (about 10E) to get stamped, then take the stamped form back to the Oficina de Extranjeros so they can give you your NIE number.